This blog, written back in 2011 and run by a hair transplant doctor, seems to think it's intentional.
First, the hairline is too straight. A natural hairline is never as
straight as Silvio’s. Second, the hairline is just too darn low. Many
individuals who wear hairpieces love to wear them in a prepubescent
position. If they would simply wear them higher ...
As explained here:
Longtime Bruce Springsteen guitarist Steven Van Zandt starred as Silvio Dante in "The Sopranos," and it was obvious from the pilot that he was sporting a mega toupee. But who was going to have guts to call him out on it? After all, he was Tony's consigliere and best buddy.
Even so, those fans longing for official confirmation of ...